Systems and methods for control of engine NOx emissions using liquid and dry reductant sources

ABSTRACT

Reductant delivery systems are disclosed that include a dry reductant source and a liquid reductant source which are operable to selectively provide gaseous reductant and liquid reductant to an exhaust aftertreatment system for treatment and reduction of NOx emissions. The gaseous reductant is provided to the exhaust aftertreatment system for treatment of NOx emissions under a first temperature condition associated with the exhaust system and the liquid reductant for treatment of NOx emissions under a second temperature condition associated with the exhaust system.

TECHNICAL FIELD

The technical field generally relates to control of engine NOxemissions, and more particularly using both liquid and dry reductantsources to control engine NOx emissions in selective catalytic reduction(SCR) systems.

BACKGROUND

Heavy duty and other diesel engine types utilize a reductant in a dieselexhaust fluid such as urea in the treatment and reduction of NOx levelsin the exhaust. The liquid urea is injected into the exhaust streamupstream of the SCR catalyst. However, when exhaust temperatures arelow, the effectiveness of liquid urea in the treatment of NOx emissionssuffers from drawbacks, such as the formation of urea deposits in theexhaust system. While superheating of the liquid urea can be used duringlow temperature conditions to obtain gaseous ammonia and reduce depositformation, these systems are costly and impractical in manyapplications.

Solid storage media systems have been developed for ammonia storage inreductant delivery systems for selective catalytic reduction (SCR). Thesolid storage media systems typically provide a dry source of reductant,such as ammonia, stored in the solid storage media that is contained ina cartridge. The ammonia is released from the solid storage media ingaseous form as needed and delivered to an exhaust gas to treatemissions in an SCR aftertreatment system. However, these systemspresent drawbacks in heavy duty engine and other high volumeapplications due to the high cost of the cartridges and the need forfrequent servicing to change the cartridge.

Therefore, there remains a need for further improvements in thearchitecture of reductant delivery systems, and in the control ofreductant delivery systems to reduce NOx emissions in low temperatureoperating conditions, that are practical for heavy duty engineapplications among others, while reducing cost and complexity. Thepresent invention meets these and other needs according to the followingdescribed embodiments.

SUMMARY

Embodiments includes unique reductant delivery systems that include adry reductant source and a liquid reductant source which are operable toselectively provide gaseous reductant and liquid reductant,respectively, to an exhaust aftertreatment system for treatment andreduction of NOx emissions. Other embodiments include unique methods,systems, and apparatus to provide gaseous reductant from a dry reductantsource to an exhaust aftertreatment system for treatment of NOxemissions under a first temperature condition associated with theexhaust system and to provide liquid reductant from a liquid reductantsource for treatment of NOx emissions under a second temperaturecondition associated with the exhaust system. In one embodiment, thetemperature condition is a liquid reductant temperature threshold. Theexhaust aftertreatment system can include an SCR catalyst that isconnected to receive exhaust gas from an internal combustion engine andthat is operable with the reductant to reduce NOx emissions.

This summary is provided to introduce a selection of concepts that arefurther described below in the illustrative embodiments. This summary isnot intended to identify key or essential features of the claimedsubject matter, nor is it intended to be used as an aid in limiting thescope of the claimed subject matter. Further embodiments, forms,objects, features, advantages, aspects, and benefits shall becomeapparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a schematic diagram of one embodiment system for providingreductant from dry and liquid reductant sources to an internalcombustion engine exhaust aftertreatment system.

FIG. 2 is a flow diagram of one embodiment of a procedure forselectively providing liquid reductant from a liquid reductant sourceand gaseous reductant from a dry reductant source to an exhaustaftertreatment system.

FIG. 3 is a schematic diagram of an apparatus for selectively providingreductant to an exhaust aftertreatment system from a dry reductantsource and a liquid reductant source.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, any alterations and further modificationsin the illustrated embodiments, and any further applications of theprinciples of the invention as illustrated therein as would normallyoccur to one skilled in the art to which the invention relates arecontemplated herein.

FIG. 1 is a schematic diagram of a system 100 for control of engine NOxemissions using a gaseous reductant and a liquid reductant. The gaseousreductant is stored in a dry solid storage media that releases thereductant in gaseous form when heated. The gaseous reductant is providedvia a metered flow to the exhaust system upstream of the SCR catalystunder a first set of temperature conditions associated with the exhaustsystem. The liquid reductant is stored in a liquid medium such as dieselexhaust fluid or urea in a storage tank or the like and provided bycontrolled injection of the liquid medium into the exhaust streamupstream of the SCR catalyst under second set of temperature conditionsassociated with the exhaust system. As used herein, an SCR catalystincludes any suitable NOx conversion catalyst. In addition, thetemperature condition of the exhaust stream can be any one orcombination of, for example, an exhaust gas temperature, an SCR catalysttemperature, an estimated catalyst or wall temperature at the locationof liquid urea dosing, or temperature of any component in the exhaust oraftertreatment system. Furthermore, the temperature condition can bedetermined by physical measurements or estimated temperatures determinedvirtually or by algorithm.

The system 100 includes an internal combustion engine 102 operable toproduce a flow of an exhaust gas stream into an exhaust system thatincludes exhaust flow path 116 and other components. In one specificembodiment, engine 102 is a diesel engine. The exhaust gas output byengine 102 includes NOx and other components which are to be reducedbefore outlet to the environment using an exhaust aftertreatment systemin exhaust flow path 116 and the dry and liquid reductant sourcesconnected to the exhaust flow path 116. System 100 is illustratedschematically and may be included with a car, truck, semi, bus, boat,recreational vehicle, construction equipment, locomotive, or other typeof vehicle. Other embodiments include an engine provided innon-vehicular applications such as a generator set.

The system 100 includes an aftertreatment system with SCR catalyst 104in exhaust flow path 116 that reduces at least a portion of the amountof NOx from the exhaust stream. System 100 also includes a gaseousreductant source 108 that stores an amount of a dry NOx reductant suchas, for example, ammonia (NH₃), in a solid storage media. In oneembodiment, the solid storage media may be any material involvingadsorption or absorption of molecular ammonia in the solid, or a solidchemical compound which can be manipulated in order to produce gaseousammonia. In one particular embodiment, the solid storage media includesmetal amine salts. The NOx reductant stored in the solid storage mediahoused in reductant source 108 may be ammonia or any other reductantunderstood in the art capable of being stored and selectively releasedfrom a solid storage media. Reductant source 108 may include a cartridgeor housing providing one or more storage units having one or morecompartments for storing ammonia in solid storage media.

System 100 also includes a first reductant delivery system 120 thatreceives gaseous reductant released from the solid storage media inreductant source 108, and provides the gaseous reductant to the exhaustflow path 116 at a position upstream of the NOx conversion catalyst 104.Gaseous reductant passes through a reductant supply line 121 fromreductant source 108 to a metering device 106 and from metering device106 to a mixer 107 connected in fluid communication with exhaust flowpath 116. The mixer 107 is located upstream of the SCR catalyst 104.Mixer 107 is supplied with gaseous reductant from reductant source 108and is operable to mix reductant gas with exhaust gas in exhaust flowpath 116. Reductant delivery system 120 may include sensors, controlvalves, heating sources, coolant lines, and other devices useful in therelease of gaseous reductant from the solid storage media and in thedelivery of the gaseous reductant to the exhaust flow path 116 in thedesired amount, rate and timing.

In one embodiment, gaseous reductant source 108 is operatively coupledwith at least one engine coolant feed line and an engine coolant returnline that provide a source of heat that heats the solid storage mediastored in reductant source 108 to release the stored reductant ingaseous form. Other embodiments contemplate other means for heating thesolid storage media in reductant source 108, including, for example, anelectrical heating element coupled to a power source such as a batteryor generator. The heat source can be embedded in the solid storagemedia, or can extend around the outside of the solid storage media, or acombination of these arrangements. In one embodiment, heating of thesolid storage material releases gaseous NH₃ from the solid storage mediainto supply line 121 by thermal desorption. The consumption rate of thereleased NH₃ gas is measured by metering device 106 as it is mixed intoexhaust flow path 116 upstream of the SCR catalyst 104.Pressure/temperature sensor 118 provide signals corresponding to thepressure of the gas released into supply line 121 and signalscorresponding to the temperature of the solid storage media in reductantsource 108 for control of the release of the reductant gas. Thetemperature and pressure signals may be provided continuously ordiscretely, and by a single device or separate devices.

System 100 also includes a liquid reductant source 110 that stores anamount of liquid NOx reductant such as, for example, ammonia (NH₃), in aliquid storage medium. In one embodiment, the liquid storage medium isdiesel exhaust fluid stored in a tank. Other liquid reductant storagemediums such as urea are also contemplated.

System 100 also includes a second reductant delivery system 122 thatreceives liquid reductant in the liquid storage medium released from theliquid reductant source 110, and provides the liquid reductant to theexhaust flow path 116 at a position upstream of the SCR catalyst 104.Liquid reductant passes through a reductant supply line 123 fromreductant source 110 to a dosing device 114 and from dosing device 114to an injector 112 connected in fluid communication with exhaust flowpath 116. The injector 112 is located upstream of the SCR catalyst 104.Injector 112 is supplied with liquid reductant from reductant source 110and is operable to inject or otherwise mix liquid reductant into exhaustflow path 116 for mixing with exhaust gas. Reductant delivery system 122may include sensors, control valves, heating sources, coolant lines, andother devices useful in the delivery of liquid reductant from thestorage source 110 to the exhaust flow path 116 in the desired amount,rate and timing.

In one embodiment, the exhaust aftertreatment system may include anoxidation catalyst 130 which is in fluid communication with exhaust flowpath 116 and is operable to catalyze oxidation of one or more compoundsin the exhaust gas flowing through exhaust flow path 116, for example,oxidation of unburned hydrocarbons or oxidation of NO to NO₂. Oxidationcatalyst 130 can be any of various flow-through oxidation catalysts.Generally, oxidation catalyst 130 includes a substrate with an activecatalyst layer configured to oxidize at least some particulate matter(e.g., the soluble organic fraction of soot) in the exhaust and reduceunburned hydrocarbons and CO in the exhaust to less environmentallyharmful compounds. For example, in some implementations, the oxidationcatalyst 130 may sufficiently reduce the hydrocarbon and COconcentrations in the exhaust to meet the requisite emissions standards.

The exhaust aftertreatment system may also include a diesel particulatefilter 132 in fluid communication with exhaust flow path 116 andoperable to reduce the level of particulates in exhaust flowing throughexhaust flow path 116. In an exemplary embodiment diesel particulatefilter 132 is a catalyzed soot filter. The diesel particulate filter 132can be any of various particulate filters known in the art configured toreduce particulate matter concentrations, e.g., soot and ash, in theexhaust gas to meet requisite emission standards. The diesel particulatefilter 132 includes a filter substrate that captures soot and otherparticulate matter generated by the engine 102. The system 100periodically regenerates diesel particulate filter 132 to removeparticulate matter that has accumulated on the diesel particulate filterover time. For example, diesel particulate filter 132 can be regeneratedby increasing the temperature of the exhaust gas above a thresholdtemperature corresponding with combustion of the particulate matter.

In certain implementations, the system 100 includes an exhaust gasrecirculation (EGR) line (not shown) configured to allow a portion ofthe exhaust gas generated by the engine to recirculate back into theengine for altering the combustion properties of the engine 102. Theexhaust aftertreatment system may further include a hydrocarbon (HC)injector (not shown) which is supplied with HC from an HC reservoir andis operationally coupled to the exhaust stream at a position upstream ofSCR catalyst 104. Other embodiments of system 100 may include engine 102having a common rail fuel system capable of injecting a post injectionfuel where at least a portion of the post injection fuel does notcombust to provide HC in the exhaust stream. Embodiments are alsocontemplated without a HC injector. Certain embodiments may also includean ammonia oxidation AMOX catalyst (not shown) at a position downstreamof the SCR catalyst 104, which is operable to catalyze the reaction ofNH₃ which slips past the SCR catalyst 104.

Reductant gas or liquid injected into exhaust flow path 116 is providedto the SCR catalyst 104 which is in flow communication with exhaust flowpath 116 and is operable to catalyze the reduction of NO_(x). SCRcatalyst 104 can be any of various catalysts known in the art. Forexample, in some implementations, the SCR catalyst with a zeolite basedcatalyst, such as a Cu-Zeolite or a Fe-Zeolite catalyst, or a vanadiumbased catalyst.

Exhaust flow path 116, as illustrated schematically in FIG. 1, may beprovided in a variety of physical configurations. In an exemplaryembodiment an exhaust flow path proceeds from the output of aturbocharger (not shown) of engine 102 through a conduit to a structurecontaining oxidation catalyst 130 and diesel particulate filter 132,through a second conduit to a structure containing the SCR catalyst 104and through another conduit which outlets to the ambient environment. Inother embodiments, the components of the exhaust gas after-treatmentsystem can be positioned in any of various arrangements, and the systemcan include other components or fewer components. Generally, exhaust gastreated in the exhaust gas after-treatment system and released into theatmosphere consequently contains significantly fewer pollutants, such asdiesel particulate matter, NOx, hydrocarbons, and carbon monoxide, thanuntreated exhaust gas.

The system 100 further includes a controller 124 that performs certainoperations for controlling reductant delivery to exhaust flow path 116from first reductant source 108 and second reductant source 110 inresponse to a temperature condition associated with the exhaust system.As used herein, a temperature condition associated with the exhaustsystem can include a temperature condition of SCR catalyst 104, atemperature condition of exhaust gas at the exhaust manifold or theconnection of engine 102 with exhaust flow path 116, a downstreamtemperature condition of the exhaust gas in exhaust flow path 116, or atemperature condition of a component of the exhaust system. Thetemperature condition can also be a combination, average, weightedaverage, of these temperature conditions or other suitable determinationof a temperature condition associated with the exhaust system determinedfrom temperature sensors, derived from temperature sensors, ordetermined from operating conditions.

In one embodiment, controller 124 is configured to determine thetemperature condition of the exhaust system is in a low temperaturerange and control first reductant source 108 to provide gaseousreductant to the exhaust flow path 116. Controller 124 is alsoconfigured to determine the temperature condition of the exhaust systemis above the low temperature range and control second reductant source110 to provide liquid reductant to the exhaust flow path 116. Thecontroller 124 may include modules structured to functionally executeoperations to determine the temperature condition and control reductantdelivery in response to the temperature condition. In certainembodiments, the controller 124 includes an exhaust system temperaturecondition module that is configured to determine the temperaturecondition associated with the exhaust system and a reductant sourceselection module configured to select reductant source 108 or reductantsource 110 in response to the temperature condition.

Controller 124 is connected to various sensors to receive or determineoperating parameters of system 100 and to provide certain controloutputs in response to the operating parameters according to programmedinstructions. In the illustrated embodiment, controller 124 is connectedto NOx sensors 140 and temperature sensors 142 to receive inputsregarding the operation of the exhaust system and the performance, suchas NOx conversion efficiency, of the aftertreatment system. Pressuresensor or differential pressure sensor 144 provides signals regardingthe pressure associated with particulate filter 132, and ammonia sensor146 provides signals regarding a mid-bed ammonia amount of SCR catalyst104 for feedback control of reductant delivery along with NOx sensors140. Additional sensors associated with the exhaust system can beprovided and are not shown, such as additional NOx and/or temperaturesensors at other locations, other ammonia sensors, and flow sensors.

Controller 124 may include one or more modules as discussed abovestructured to functionally execute the operations described herein. Thedescription herein, including modules, emphasizes the structuralindependence of the aspects of the controller 124, and illustrates onegrouping of operations and responsibilities of the controller 124. Othergroupings that execute similar overall operations are understood withinthe scope of the present application. Modules may be implemented inhardware and/or software on computer readable medium, and modules may bedistributed across various hardware or software components.

Controller 124 forms a portion of a processing subsystem including oneor more computing devices having memory as well as a number of inputsand outputs for interfacing with various sensors and subsystems ofsystem 100. Controller 124 can include an electronic circuit comprisedof one or more components, including digital circuitry, analogcircuitry, or both. Controller 124 may be a single device or adistributed device. Controller 124 may include one or more controlalgorithms defined by operating logic in the form of softwareinstructions, hardware instructions, firmware instructions, dedicatedhardware, or the like.

In one form, controller 124 is of a programmable microcontrollersolid-state integrated circuit type that includes memory and one or morecentral processing units. The memory of controller 124 includes of oneor more components and can be of any of volatile or nonvolatile,solid-state, optical media, magnetic media, combinations of these, orother types of memory. Controller 124 can include signal conditioners,signal format converters (such as analog-to-digital anddigital-to-analog converters), limiters, clamps, filters, and the likeas needed to perform various control and regulation operations describedherein. Controller 124, in an exemplary embodiment, may be a type ofcontroller sometimes referred to as an electronic or engine controlmodule (ECM), electronic or engine control unit (ECU) or the like, thatis directed to the regulation and control of engine operation.Alternatively, controller 124 may be dedicated to the control of justthe operations described herein or to a subset of controlled aspects ofsystem 100.

An exemplary procedure to be performed by controller 124 for determininga temperature condition and reductant source selection for delivery ofreductant to exhaust flow path is described in FIG. 2. In FIG. 2,procedure 200 includes an operation 202 to determine an operatingcondition of engine 102. Engine operating condition determinations caninclude determining operating conditions that produce exhaust gas torespond to a torque request or to satisfy a torque demand, thatcorrespond to an engine start-up request or start-up condition, thatcorrespond to an engine shut-down request or condition, or thatcorrespond to an eminent engine shut-down condition.

Procedure 200 further includes an operation 204 to determine atemperature condition associated with the exhaust system. As discussedabove, the temperature condition can correspond to a temperature of theexhaust gas at one or more locations, a temperature of SCR catalyst 104,a temperature condition of a component of the exhaust system, or acombination of these. The temperature condition can be determined from asingle input from a temperature sensor or combination of sensors, anaverage or other determination based on a number of inputs from one ormore temperature sensors, an input from a virtual temperature sensor, ora derived or calculated value from operating conditions.

Procedure 200 continues at conditional 206 to determine if thetemperature condition associated with the exhaust system is greater thanor equal to a liquid reductant threshold temperature. The liquidreductant threshold temperature can be, for example, a temperature at orabove which liquid reductant is effectively hydrolyzed in the exhaustgas to prevent or reduce deposit formation in the exhaust system. Otherliquid reductant threshold temperatures are also contemplated and notprecluded.

If conditional 206 is negative, procedure 200 continues at operation 208and provides the gaseous reductant from gaseous reductant source 108 tothe exhaust system. If conditional 206 is positive, procedure 200continues at operation 210 and provides liquid reductant from liquidreductant source 110 to the exhaust system.

Providing gaseous reductant when the temperature condition is less thanthe liquid reductant temperature threshold enables treatment of NOxemissions in low temperature operating conditions while avoiding orminimizing the formation of reductant deposits typically formed bydosing of liquid reductant in low temperature conditions. Since thefirst reductant source 108 including the solid storage media is usedonly during certain operating conditions, the stored amount of reductantin the solid storage media can be significantly less than what would berequired if the solid storage media stored the entire reductant supplyfor all operating conditions. Thus, in one embodiment, first reductantsource 108 includes replaceable cartridges of solid storage media withstored dry reductant that are removed and replaced with anothercartridge when empty.

In other embodiment of procedure 200, the engine operating conditiondetermination 202 and/or exhaust temperature condition determination 204can include a reductant storage capacity determination of the SCRcatalyst 104 and an eminent engine shut-down condition determination. Ifstorage capacity on SCR catalyst 104 is available, an eminent engineshut-down condition determination can result in an operation to doseliquid reductant from second reductant source 110 to store reductant onSCR catalyst 104 for NOx reduction in a subsequent engine cold startwhile the catalyst temperature is still above the liquid reductantdosing threshold.

In another embodiment, the engine operating condition determination 202and/or exhaust temperature condition determination 204 can include areductant storage capacity determination of the SCR catalyst 104 and anengine shut-down condition determination. If storage capacity on SCRcatalyst 104 is available, an engine shut-down condition determinationcan result in an operation to provide gaseous reductant from firstreductant source 108 to store reductant on SCR catalyst 104 for NOxreduction in a subsequent engine cold start while the catalysttemperature is below the liquid reductant dosing threshold.

In another embodiment, the engine operating condition determination 202and/or exhaust temperature condition determination 204 can include areductant storage capacity determination of the SCR catalyst 104 and anengine start-up condition determination. The start-up condition can bedetermined based on, for example, ambient conditions, a door switchactivation, a key-switch activation, a key-on delay timing, atime-of-day, a day of week, a preprogrammed schedule, an operator input,or other start-up indicator. If storage capacity on SCR catalyst 104 isavailable, an engine start-up determination can result in an operationto provide gaseous reductant from first reductant source 108 to storereductant on SCR catalyst 104 for NOx reduction in the subsequent enginestart-up while the catalyst temperature is below the liquid reductantdosing threshold. In one embodiment, gaseous reductant from firstreductant source 108 is provided upon engine start-up following apre-determined amount of key-off time when ambient temperature is belowa pre-determined threshold.

In still other embodiments, the controller 124 is configured to use onlyone of reductant sources 108, 110 when the other reductant source isdetermined to be unavailable due to, for example, being empty ormalfunctioning. In one embodiment, an indicator is provided to thedriver when one of the reductant sources is empty or malfunctioning, butoperation of the vehicle is maintained by providing reductantexclusively from the other reductant source until service is completed.In yet other embodiments, controller 124 is configured to use dryreductant source 108 exclusively in response to an estimated ureadeposit build-up or accumulation in the aftertreatment system exceedinga predetermined limit, or in response to a determination of a freezingcondition associated with liquid reductant source 110 or any of itscomponents connecting it the exhaust system. The determination of ureadeposit thresholds or limits and frozen or potentiallyfrozen/stuck/blocked conditions can be performed by any known technique.The determination of the freezing condition can be a determination of acold weather condition likely to cause a freezing condition, of anactual frozen condition, or of a blocked condition associated with atemperature of the liquid reductant or a blockage preventing or reducingflow of the liquid reductant. In further embodiments, reductant fromboth first reductant source 108 and second reductant source 110 is usedsimultaneously under certain operating conditions, such as during atransition from supplying reductant only from the dry reductant sourceto supplying reductant only from the liquid reductant source.

The schematic flow diagrams and related descriptions above provideillustrative embodiments of performing procedures for selective gaseousreductant and liquid reductant delivery in response to a temperaturecondition of the exhaust system. Operations illustrated are understoodto be exemplary only, and operations may be combined or divided, andadded or removed, as well as re-ordered in whole or part, unless statedexplicitly to the contrary herein. The operations of the variousembodiments can also be combined as a single embodiment. Certainoperations illustrated may be implemented by a computer executing acomputer program product on a non-transient computer readable storagemedium, where the computer program product comprises instructionscausing the computer to execute one or more of the operations, or toissue commands to other devices to execute one or more of theoperations.

Referring to FIG. 3, an apparatus including controller 124 includes anexhaust system temperature condition module 300 structured to determinea temperature condition associated with the exhaust system. Thecontroller 124 also includes a reductant source selection module 302structured to control the selection and dosing of gaseous reductant fromthe first reductant source 108 and liquid reductant from the liquidreductant source 110 in response to the temperature conditiondetermination.

Exhaust system temperature condition module 300 is structured to receiveone or more temperature inputs 306 associated with the exhaust systemand provide an exhaust system temperature condition output 308. In oneembodiment, the exhaust system temperature condition module 300 isfurther structured to determine the temperature condition in response toan imminent engine shut-down or completed engine shutdown. In yet afurther embodiment, the exhaust system temperature condition module 300is structured to determine the temperature condition in response to anengine start-up condition.

The reductant source selection module 302 is structured to receive andinterpret the temperature condition output 308 and provide a reductantsource selection command 312 that controls providing gaseous reductantfrom the first reductant source 108 if the temperature condition 308 isless than a liquid reductant temperature threshold 310 and providingliquid reductant from the second reductant source 110 if the temperaturecondition 308 is greater than or equal to the liquid reductanttemperature threshold 310. In further embodiments, reductant sourceselection module 302 is structured to select reductant source 108 orreductant source 110 based on the temperature condition output 308 inresponse to an eminent engine shut-down condition, an engine shut-downcondition, or engine start-up condition based on a reductant storagecapacity of SCR catalyst 104. Other embodiments contemplate reductantsource selection module 302 is structured to select one of reductantsource 108, 110 in response to an empty or malfunction condition of theother reductant source, and to select reductant source 108 in responseto a urea deposit condition or a freezing condition associated withsecond reductant source 110.

As is evident from the figures and text presented above, a variety ofembodiments according to the present disclosure are contemplated.Example of the contemplated embodiments are provided in the claimsappended hereto, but are not limited to the claims.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly certain exemplary embodiments have been shown and described. Thoseskilled in the art will appreciate that many modifications are possiblein the example embodiments without materially departing from thisinvention. Accordingly, all such modifications are intended to beincluded within the scope of this disclosure as defined in the followingclaims.

In reading the claims, it is intended that when words such as “a,” “an,”“at least one,” or “at least one portion” are used there is no intentionto limit the claim to only one item unless specifically stated to thecontrary in the claim. When the language “at least a portion” and/or “aportion” is used the item can include a portion and/or the entire itemunless specifically stated to the contrary.

What is claimed is:
 1. A method, comprising: producing an exhaust gas inan exhaust flow path including a selective catalytic reduction (SCR)catalyst, wherein producing the exhaust gas includes operating aninternal combustion engine; determining a temperature condition of theexhaust gas; determining an engine shut-down condition, wherein theengine shutdown condition includes a pre-shutdown condition and apost-shutdown condition; and selectively providing one of a gaseousreductant released from a dry reductant source and a liquid reductantfrom a liquid reductant source to the exhaust flow path upstream of theSCR catalyst in response to the engine shutdown condition, wherein thegaseous reductant is provided from the dry reductant source in responseto the post-shutdown condition and the liquid reductant is provided fromthe liquid reductant source in response to the pre-shutdown condition.2. The method of claim 1, wherein determining the temperature conditionof the exhaust gas includes determining a temperature of the SCRcatalyst.
 3. The method of claim 1, wherein the liquid reductant sourceincludes a urea solution in a storage tank.
 4. The method of claim 1,wherein the gaseous reductant includes ammonia and the dry reductantsource includes a solid storage media.
 5. The method of claim 1, whereinthe exhaust flow path includes a diesel oxidation catalyst and a dieselparticulate filter upstream of the SCR conversion catalyst.
 6. Themethod of claim 1 further comprising: providing the gaseous reductantreleased from the dry reductant source in response to an engine start-upcondition before operating the internal combustion engine to produce theexhaust gas.
 7. The method of claim 1, wherein providing the gaseousreductant from the dry reductant source includes heating the dryreductant source.
 8. The method of claim 1, wherein selectivelyproviding one of the gaseous reductant and the liquid reductant includesdetermining that one of the dry reductant source and liquid reductantsource is unavailable and selecting the other of the dry reductantsource and the liquid reductant source.
 9. A method, comprising:producing an exhaust gas in an exhaust flow path including a selectivecatalytic reduction (SCR) catalyst; determining a temperature conditionof the exhaust gas, wherein determining the temperature conditionincludes determining an amount of key-off time; and selectivelyproviding one of a gaseous reductant released from a dry reductantsource and a liquid reductant from a liquid reductant source to theexhaust flow path upstream of the SCR catalyst in response to thetemperature condition of the exhaust gas, wherein the gaseous reductantis provided when the key-off time exceeds a predetermined amount of timeand an ambient temperature is below a predetermined threshold.
 10. Themethod of claim 9, wherein determining the temperature condition of theexhaust gas includes determining a temperature of the SCR catalyst. 11.The method of claim 9, wherein the liquid reductant source includes aurea solution in a storage tank.
 12. The method of claim 9, wherein thegaseous reductant includes ammonia and the dry reductant source includesa solid storage media.
 13. The method of claim 9, wherein the exhaustflow path includes a diesel oxidation catalyst and a diesel particulatefilter upstream of the SCR conversion catalyst.
 14. The method of claim9, wherein providing the gaseous reductant from the dry reductant sourceincludes heating the dry reductant source.
 15. The method of claim 9,wherein selectively providing one of the gaseous reductant and theliquid reductant includes determining that one of the dry reductantsource and liquid reductant source is unavailable and selecting theother of the dry reductant source and the liquid reductant source. 16.The method of claim 9 further comprising: providing the gaseousreductant released from the dry reductant source in response to anengine start-up condition before operating the internal combustionengine to produce the exhaust gas.
 17. A system, comprising: an internalcombustion engine operable to produce an exhaust stream, the exhauststream including an amount of NO_(x) emitted into an exhaust flow path;a first reductant source that stores a NO_(x) reductant in a solidstorage media and a second reductant source that stores the NO reductantin a liquid medium; a selective catalytic reduction (SCR) conversioncatalyst in the exhaust flow path, wherein the first reductant sourceand the second reductant source are each connected to the exhaust flowpath upstream of the SCR catalyst to provide the NO_(x) reductant to theSCR catalyst; and a controller connected to one or more sensorsassociated with the exhaust stream operable to indicate a temperaturecondition of the exhaust stream, wherein the controller is configuredto: determine an engine shut-down condition of the internal combustionengine, wherein the engine shutdown condition includes a pre-shutdowncondition and a post-shutdown condition, control the first reductantsource to dose the NO_(x) reductant in a gaseous form from the solidstorage media into the exhaust stream in response to the post-shutdowncondition, and control the second reductant source to dose the NO_(x)reductant from the liquid medium in a liquid form in response to thepre-shutdown condition.
 18. The system of claim 17, wherein the firstreductant source doses the NO_(x) reductant in the gaseous form inresponse to heating of the solid storage media.
 19. The system of claim17, wherein the first reductant source is connected to the exhaust flowpath with a supply line that includes a metering device to provide ameasured flow rate of the gaseous reductant to the exhaust flow path.20. The system of claim 17, wherein the NO_(x) reductant comprisesammonia, the solid storage media comprises metal amine salts, and theliquid storage medium comprises a urea solution.
 21. The system of claim17, wherein the exhaust flow path includes a diesel oxidation catalystand a diesel particulate filter upstream from the SCR catalyst.
 22. Thesystem of claim 17, wherein the first reductant source includes areplaceable cartridge containing the solid storage media mounted to achassis of a vehicle powered by the internal combustion engine and thesecond reductant source includes a storage tank mounted to the chassis.23. An apparatus, comprising: an electronic controller structured toreceive operating parameters from at least one sensor associated with anexhaust system that is connected with a first reductant source forstoring a gaseous reductant in a solid storage media and a secondreductant source for storing a liquid reductant in a liquid storagemedium, wherein the controller includes: an exhaust system temperaturecondition module structured to determine a temperature conditionassociated with the exhaust system and determine the temperaturecondition in response to a shutdown of an internal combustion engineconnected to the exhaust system, wherein the temperature conditionincludes a pre-shutdown condition and a post-shutdown condition; and areductant source selection module structured to selectively dose gaseousreductant from the first reductant source and liquid reductant from thesecond reductant source in response to the temperature condition and toprovide gaseous reductant from the first reductant source in response tothe post-shutdown condition and to provide liquid reductant from thesecond reductant source in response to the pre-shutdown condition. 24.The apparatus of claim 23, wherein the exhaust system temperaturecondition module is further structured to determine the temperaturecondition in response to a startup condition of an internal combustionengine connected to the exhaust system and the reductant sourceselection module is further structured to provide gaseous reductant fromthe first reductant source in response to the startup condition.
 25. Theapparatus of claim 23, wherein the reductant source selection module isstructured to dose gaseous reductant from the first reductant source andliquid reductant from the second reductant source under certainoperating conditions.